Watch size

I would say, my 6xPS has 60/61mm.

Ok that quite a difference and would stick out over my arm. Guess I'll really have to stick to the medium size. :D

My f6X is 56.9mm.

Yes, thanks.

I just measured my old F5 47mm, it has a length of 58mm, I think I was ok with that, so maybe a F7X is something for me...

Update. I did buy the 6 pro (47mm, my wrist size 160mm) and got used to it very quickly. I think the 51mm would work also. If you want it enough, it'll work!

Here are a couple of images of an f5+ and an f6X on the same wrist:

Although the f6X is a few mm wider than the f5+ it sits better on my odd shaped wrists

I wonder what the size of the F7X will be, maybe a little shorter than the F6X, but even with the same length as the 6 series it's definitely smaller than the old F5 47mm standard edition. Soo tempting. 

According to the specifications on the Garmin (en-GB) website the f7X has the same dimensions as the f6X.

The watch strap bar-to-bar distance on an f5 or f5+ are about the same as that of the f6X. If the f5 47mm fits you then an f6X or f7X will most likely fit you.

Other than the f6X being a few grams heavier, the f5+ (47mm) and f6X (51mm) wear the same for me.

The f5+ (47mm) has the same case dimensions as the f5 (47mm). Garmin made the bar-to-bar distance shorter on the f6 series by changing from screw type bars to spring bars for the strap attachments allowing the lugs to become shorter.